Ondetti et al. in U.S. Pat. No. 4,105,776 discloses compounds of the formula ##STR4## including those wherein the proline ring is substituted with a hydroxy or lower alkyl group as useful in reducing or relieving angiotensin related hypertension.
Cushman et al. in U.S. Pat. No. 4,052,511 discloses compounds of the formula ##STR5## including those wherein the proline ring is substituted with a hydroxy or lower alkyl group as being useful as angiotensin converting enzyme inhibitors.
Ondetti et al. in U.S. Pat. No. 4,154,935 discloses compounds of the formula ##STR6## wherein R.sub.2 and R.sub.2 ' are halogen or hydrogen as being useful as hypotensive agents.
Iwao et al. in U.K. Patent Application, GB 2027025 A published Feb. 13, 1980 disclose antihypertensive agents of the formula ##STR7## wherein R.sub.1 is phenyl or hydroxy substituted phenyl.
Ondetti et al. in U.K. Patent Application GB 2028327 A published Mar. 5, 1980 disclose hypotensive agents of the general formula ##STR8## wherein X is oxygen or sulfur and R.sub.1 is lower alkyl, phenyl, or phenyl-lower alkyl.
Krapcho in U.S. Pat. No. 4,217,359 discloses compounds of the formula ##STR9## wherein R.sub.0 and R.sub.1 can be hydrogen or lower alkyl.
Krapcho in U.K. Patent Application GB 2,039,478 A and U.S. Ser. No. 99,164 disclose mercaptoacyl derivatives of proline wherein the proline ring has a diether; dithioether, ketal or thioketal substituent in the 4-position.